Mesh scaling linter error

[stephane-caron]

My linter reports an undefined name in _link_mesh:

    def _link_mesh(self, link, collision_geometry=True):
        # ...
        for g in geometries:
            for m in g.geometry.meshes:
                # ...
                    if g.geometry.mesh.scale is not None:
                        S[:3, :3] = np.diag(c.geometry.mesh.scale)  # undefined c?

This would trigger a NameError on a URDF with a scaled mesh.